I'll add a config setting for the next release so you will be able to add this in user.config to turn off those log events:
<add key="LogBlockedRequests" value="false" />
If you put that in user.config then after the next upgrade it will stop logging those blocked requests.
The only other options for now is you could change the log level in log4net.config from INFO to WARN which would turn off all the informational logging but still log errors.
Or instead of using the banned ip support inside mojoPortal just block those ip addresses at the windows firewall instead, that blocks them from ever reaching your site or any other sites on your server.